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Two systems, in which the frequency of a high quality quartz crystal oscillator can be controlled by a servo system employing as a reference frequency the (F = 4, m F = 0)↔(F = 3, m F = 0) transition in the ground electronic state of cesium 133 , have been in operation for about one year at the National Bureau of Standards. These systems are presently used in conjunction with the United States Frequency Standard, NBS II, and the alternate standard, NBS I, for measuring the frequencies of the United States Working Frequency Standards on a regular basis. The dependability, precision, and accuracy of the servo-derived measurements have been compared with the corresponding figures for the more direct manual method. Although both measurement systems have been found to be highly dependable, the servo method has significant advantages with respect to convenience of operation and measurement precision. These advantages can be utilized with no sacrifice of accuracy. Typical servo measurement precision is 2×10 -12 for a 30-minute averaging time, while the measurement accuracy for both methods is 1.1×10 -11 . For longer measurement periods of 12-14 hours, precisions and reproducibilities of 2×10 -13 have been observed.
Beehle et al. (Sat,) studied this question.